REPTILES, BIRDS, AND MAMMALS 



465 



the second and fifth digits were lost, leaving the typical two-toed condi- 

 tion seen in the cloven-hoofed pigs, camels, deer, giraffes, and cattle. By 

 lifting the hand on the axis of the middle finger one may imitate the 

 changes that occurred in the odd-toed perissodactyls. First there was loss 

 of the thumb and then of the little finger, leaving a three-toed foot such 

 as is found in tapirs, rhinoceroses, and most of the fossil horses. In later 

 horses this was followed by loss of the second and fourth digits, giving 

 the ultimate in toe reduction, the one-toed foot. 



Characteristics of the modern horse. The nature of the evolutionary 

 changes in the horse group will be most easily comprehended if we first 

 consider the principal adaptive features of the familiar horse of today. 



Fig. 28.31. The skull of a modern Arabian horse. {Courtesy American Museum of Natural 

 History.) 



The horse is a large, swift-footed grazing animal. Its most distinctive 

 characters are those of the teeth and feet. The skull is long and consists 

 mostly of face; the part in front of the eyes is extended far forward to 

 accommodate a powerful battery of grinding cheek teeth and a set of 

 cropping teeth in front. The grinding teeth include the molars and all 

 but the first of the premolars; they are all much alike — square prisms 

 with a complex pattern of infolded enamel on the grinding surface. These 

 teeth are tall, with high crowns and short roots, and are set deep in the 

 jaws; the rear part of the lower jaw is much deepened to accommodate 

 them. The grinding surface is worn away by the chewing of abrasive 

 foods, and the nearly vertical harder enamel layers form little ridges sup- 

 ported by the inner dentine of the tooth on one side and by cement on 

 the other. Cement is a limy material deposited on the surface of the tooth 

 and between its folds by secretions of the mouth. The cheek teeth con- 



